1. Field of the Invention
This invention relates to shadow mask type color cathode ray tubes, and, more particularly, to the shadow mask.
2. Discussion of Background
In general, a shadow mask type color cathode ray tube comprises an electron gun in the tube emitting three electron beams, a shadow mask distributing these beams selectively by color, and a phosphor screen emitting light in the three colors, red, green and blue, on excitation by these beams. The image formed on the screen is observed through an envelope panel. In the shadow mask there are provided a large number of apertures which correspond precisely with the phosphor pattern of the respective color on the screen. As the effective electron beams passing through these apertures during color cathode ray tube operation represent somewhat less than a third of the incoming beams, the rest of the electrons impinge on the shadow mask and their energy is converted into heat energy, raising the temperature of the shadow mask. In a normal operating television set, the shadow mask is thereby heated to a temperature of about 80.degree. C. In the special color cathode ray tubes used in the instrument panels in aircraft cockpits, the shadow mask temperature can rise to around 200.degree. C. Most shadow masks consist of a lamina 0.1 to 0.3 mm thick, made by cold rolling, of which the main constituent is iron of thermal expansion coefficient 1.2.times.10.sup.-5 /.degree.C. The rigid L section mask frame that supports the shadow mask skirt is about 1 mm thick, is likewise made by cold rolling, and is subjected to blackening treatment. Thermal expansion readily occurs when the shadow mask is heated. Since the shadow mask periphery is adjacent to the blackened mask frame, which has a large heat capacity, heat is transferred from the shadow mask periphery to this mask frame by radiation or conduction. This results in the temperature of the shadow mask periphery falling below the temperature at its center, producing a temperature difference between the center and periphery. This produces the "doming" phenomenon caused by relative thermal expansion taking place principally at the center. Consequently the distance between the shadow mask and phosphor screen alters, disturbing the accurate landing of the electron beams and thus impairing color purity. This phenomenon of mislanding due to doming is particularly evident when the color cathode ray tube has just been switched on. Also, if part of the picture is locally of high luminance and especially if such high luminance portions are stationary for some time, high electron flow density regions occur on the shadow mask, causing local doming.
With regard to this doming phenomenon in color cathode ray tubes, there have been a number of proposals aimed at promoting dispersal of heat from the center of the shadow mask. For instance, in U.S. Pat. No. 2826538 (Hunter et al.), it is proposed to facilitate shadow mask heat dispersal by providing a black layer of graphite on the shadow mask surface. Such a graphite layer in the color cathode ray tube acts as an excellent radiator, lowering the shadow mask temperature. However, such a black graphite layer has the following drawbacks. The thermal cycle of the heating process involved in the manufacture of the color cathode ray tube impairs the adhesion of the black layer so that when the color cathode ray tube is subjected to vibration, part of this layer separates and minute flakes fall off. When this happens, flakes adhering to the shadow mask cause blockage of the electron apertures, adversely affecting the characteristics of the image on the phosphor screen. Flakes adhering to the electron gun cause sparks between the electrodes, impairing the withstand voltage characteristic, and so forth, so that the quality of the color cathode ray tube is markedly reduced.
It has been proposed, in Japanese Patent Application No. 58-148843 (Disclosure No. 60-54139), to control doming by using high temperature heat treatment to seal lead borate glass to the surface of the shadow mask. However, since this glass layer, which is bonded to the surface of the shadow mask, contains a great deal of lead (which has a very high atomic number), it is difficult to reduce the elastic reflection of the electrons impinging on the shadow mask. In Japanese Patent Publication No. 49-14777, a proposal was made to prevent such electron scattering by nickel plating the vicinity of the mask apertures. However, the method of manufacture is not practical because it is too complicated, and electron scattering by the surface of the shadow mask apart from the apertures cannot be altogether eliminated. Electron scattering causes emission of light from undesired parts of the screen, spoiling image contrast, and lowering color purity.